The present invention relates generally to an apparatus and method for flotation separation of mineral particles including coal and similar materials, and more particularly pertains to an improved apparatus and method for beneficiating coal by flotation separation of a froth generated by a pressure-reducing spray nozzle such that ground coal particles may be separated from impurities associated therewith such as ash and sulfur.
Coal is an extremely valuable natural resource in the United States because of its relatively abundant supplies. It has been estimated that the United States has more energy available in the form of coal than in the combined natural resources of petroleum, natural gas, oil shale, and tar sands. Recent energy shortages, together with the availability of abundant coal reserves and the continuing uncertainties regarding the availability of crude oil, have made it imperative that improved methods be developed for converting coal into a more useful energy source.
Many known prior art processes for froth flotation separation of a slurry of particulate matter are based on constructions wherein air is introduced into the liquid slurry of particulate matter, as through a porous cell bottom or a hollow impeller shaft, thereby producing a surface froth. These prior art methods are relatively inefficient approaches, especially when large amounts of particular matter are being processed. Generally, these techniques are inefficient in providing sufficient contact between the particulate matter and the frothing air. As a result, they require relatively large amounts of energy to generate the froth. In addition, froth flotation techniques which permit bubbles to rise in the slurry can tend to trap and carry impurities such as ash in the froth slurry, and accordingly the resultant beneficiated particulate product frequently has more impurities therein than desired.
Methods have been suggested and are being explored in the beneficiation of coal, i.e., the cleaning of coal of impurities such as ash and sulfur, either prior to burning the coal or after its combustion. In one recently developed technique for beneficiation, termed herein chemical surface treating, raw coal is pulverized to a fine mesh size and is then chemically treated. According to this technique, the treated coal is then separated from ash and sulfur, and a beneficiated or cleaned coal product is recovered therefrom. In further detail, in the heretofore mentioned chemical surface treating process, coal is first cleaned of rock and the like, and is then pulverized to a fine size of about 48 to 300 mesh. The extended surfaces of the ground coal particles are then rendered hydrophobic and oleophilic by a polymerization reaction. The sulfur and mineral ash impurities present in the coal remain hydrophilic and are separated from the treated coal product in a water washing step. This step utilizes oil and water separation techniques and results in hydrophobic coal particles floating upon a water phase which contains hydrophilic impurities.
In greater detail, commonly assigned McGarry et al. U.S. Pat. No. 4,347,126 and Duttera et al. U.S. Pat. No. 4,347,127 disclose the flotation separation of coal particles from impurities associated therewith such as ash and sulfur. In these arrangements, a primary spray hollow jet nozzle is positioned above a flotation tank having a water bath therein, and sprays an input slurry through an aeration zone into the surface of the water. The spraying operation creates a froth on the water surface in which a substantial quantity of particular matter floats while other components of the slurry sink into the water bath. A skimming arrangement skims the froth from the water surface as a cleaned, or beneficiated, product. A recycling operation is also provided wherein particulate materials which do not float after being sprayed through the primary spray nozzle are recycled to a further recycle, hollow jet spray nozzle to provide a second opportunity for recovery of the recycled particles.
Commonly assigned McGarry et al. U.S. Pat. No. 4,514,291 and copending McGarry et al. U.S. patent application Ser. No. 707,664 filed Mar. 4, 1985 disclose an improved spiral spray nozzle which does much to overcome problems associated with the full jet spray nozzle described in aforesaid U.S. Pat. Nos. 4,347,126 and 4,347,127. The full jet nozzle is characterized by a multiplicity of small apertures which results in the development of a substantial back pressure across each nozzle during its operation. Several problems have arisen with this particular nozzle design, including a recurring problem with clogging. Tank covers, filter systems, larger nozzles and extreme care and frequent cleaning are necessary to alleviate this problem. The improved open spiral nozzle of U.S. Pat. No. 4,514,291 and copending Ser. No. 707,664 which represents a significant improvement over the aforesaid full jet nozzle is available from several different manufacturers in many different types of materials including polypropylene and tungsten carbides.
A variety of arrangements for deflecting or distributing particulate material, liquids, slurries, suspensions, and the like, are known, e.g., from U.S. Pat. Nos. 2,525,025; 2,639,947; 2,712,962; 2,713,895; 3,811,620; 4,239,424; 4,350,302 and 4,529,337. However, none are known to have been provided for a spray unit utilized in a froth flotation procedure such as described in aforesaid U.S. Pat. Nos. 3,347,126 and 4,347,127.